I posed the question in the title of this post toward the end of my first class session of the exciting new seminar I am teaching this Fall 2013 semester at OSU's Moritz College of Law. Indeed, I challenged the terrific (and seemingly very insightful and knowledgeable) students taking the seminar to spend the next week looking for (and sending to me for posting here) the very best statistics and the strongest research and arguments they could find to support marijuana prohibition.

Helpfully, and to provide a running start on a question that I hope will generate some dialogue in the comments, there are more than a few anti-drug advocacy groups which have already marshalled materials in support of prohibition. A research assistant helped me assemble links to these notable advocacy groups:

I am hoping that seminar students and other readers might help me cull through the materials on these sites and elsewhere to help provide a sophisticated and detailed answer to the question in the title of this post.

UPDATE: So far, three students from my seminar have sent me a bunch of materials and links concerning what they perceive to be the "best" ideas and arguments to support marijuana prohibition. Here are a few highlights from these efforts:

The DEA in 2010 produced this impressive 81-page booket, titled Speaking Out Against Drug Legalization, which says it is "designed to cut through the current fog of misinformation with hard facts [by] present[ing] an accurate picture of America’s
experience with drug use, the nature of the drug problem, and the potential for damage if the United States adopts a more permissive policy on drug abuse." Here are a few (of dozens) of the bullet points from this document discussing the health risks posed by marijuana:

Harvard University researchers report that the risk of heart attack is five times higher than usual in the hour after smoking marijuana.

The National Institute of Health found that a person who smokes five joints per week may be
taking in as much tar and cancer-causing chemicals into their lungs as someone who smokes
a pack of cigarettes every day.

According to several recent studies, marijuana use has been linked with depression and suicidal
thoughts, in addition to schizophrenia. These studies report that weekly marijuana use among teens
doubles the risk of developing depression and triples the incidence of suicidal thoughts.

I have not sought to check or question the references cited for these statements, but I am inclined to take the claims at face value. In addition, building largely on these sorts of medical claims while making a few other arguments, here are links to a couple notable and prominent commentaries making the case in favor of marijuana prohibition:

Last but certainly not least, the Director of the Office of National Drug Control Policy, R. Gil Kerlikowske, gave this notable public statement to California Police Chiefs Association Conference in March 2010 titled "Why Marijuana Legalization Would Compromise Public Health and Public Safety." Here is how he summed up his main arguments against marijuana legalization at the start and tail end of this speech:

The concern with marijuana is not born out of any culture-war mentality, but out of what the science tells us about the drug’s effects.
And the science, though still evolving, is clear: marijuana use is harmful. It is associated with dependence, respiratory and mental illness, poor motor performance, and cognitive impairment, among other negative effects....

Legalizing marijuana would also saddle government with the dual burden of regulating a new legal market while continuing to pay for the negative side effects associated with an underground market whose providers have little economic incentive to disappear....

Legalization means the price comes down, the number of users goes up, the underground market adapts, and the revenue gained through a regulated market will never keep pace with the financial and social cost of making this drug more accessible.